These days, a semiconductor memory is widely used in a main memory of a large-scale computer, a personal computer PC, home electric appliances, portable phone and the like. As a type of the semiconductor memory, volatile dynamic RAM (DRAM) and static RAM (SRAM), nonvolatile maskROM (MROM) and flash EEPROMs such as a NAND flash and a NOR flash are commercially available. Although the DRAM is the volatile memory, this is excellent in a low cost (cell area is smaller than one-quarter of that of SRAM), high speed (faster than flash EEPROM), so that this has a large share in a personal computer (PC) market and a portable phone market.
On the other hand, a rewritable nonvolatile flash EEPROM, which protects data during power off, begins to have an enormous market share in a field of the portable phone, various cards, SSD and the like recently. However, the number of rewriting (W/E number) thereof is only 10 cubic to 10 to the power of 6, write time of microsecond or millisecond is required, and a high voltage of 12 to 22 V is required, so that this has a problem in miniaturization and function.
On the other hand, as an emerging memory, a ferroelectric memory (FeRAM), a magnetoresistance memory (MRAM), the phase change memory (PRAM), the resistance change memory (RRAM) and the like have been lively developed recently. Out of them, the magnetoresistance memory (MRAM), the phase change memory (PRAM), and the resistance change memory (RRAM) store information by changing resistance of an information memory element of a memory cell.
The magnetoresistance memory (MRAM) has a small resistance change ratio of 200% and this has a problem of a demagnetizing field, so that scaling is difficult; however, the phase change memory (PRAM) and the resistance change memory (RRAM) might replace the NOR flash and the NAND flash because a resistance value thereof changes by two digits to five digits and it is possible to realize a multi-level, and because the write current and the read current may be decreased by reduction in a memory element size and this is suitable for high integration.